1. Field of the Invention
The present invention disclosed in this specification relates to organometallic complexes which can emit light by current excitation. In particular, the present invention relates to organometallic complexes which can emit light from a triplet excited state. The present invention also relates to a light-emitting element, a display device, an electronic device, a light-emitting device, and a lighting device each of which includes any of the organometallic complexes.
2. Description of the Related Art
It has been reported that in a light-emitting element including a layer containing a light-emitting material (light-emitting layer) between a pair of electrodes (anode and cathode), a variety of organic compounds can be used as the light-emitting material.
The light-emitting element is said to have the following light emission mechanism: when voltage is applied between the pair of electrodes with the light-emitting layer interposed therebetween, electrons injected from the cathode and holes injected from the anode are recombined in the light-emitting layer to form molecular excitons, and the molecular excitons release energy to emit light when returning to a ground state. Singlet excitation and triplet excitation are known as excited states, and light emission can be obtained through either of the excited states.
In such a current-excitation light-emitting element, more excitons are generated in a triplet excited state than in a singlet excited state; thus, the emission efficiency of the light-emitting element can be increased by using a material which can emit light from the triplet excited state (phosphorescent material). Therefore, many attempts have been made to use a phosphorescent material as a light-emitting material.
As a typical example of a phosphorescent material emitting green to blue light, a metal complex in which a ligand having a heterocyclic ring skeleton is coordinated to iridium (Ir) that is a central metal is given (e.g., see Patent Document 1). In Patent Document 1, an iridium complex having a triazole derivative as a ligand is disclosed.